1. Field of the Invention
The present invention relates to a device for mounting an optical rear projection screen and for holding two parts of an optical rear projection screen together.
2. Background of the Invention
There are two ways of showing images on a screen. The most common way is front projection such as in a movie theatre. The second way is rear projection such as in a microfilm reader in a public library. The advantage of rear projection is that it permits the showing of projected images in a room with a fairly high level of ambient illumination, such as a workplace or a conference room. With the advent of electronic imaging sources (video projectors) and their seemingly endless access to different types of information, there is an increasing demand for large rear projection screen facilities.
Until recently, rear projection screens were simple affairs that consisted of a substrate material that was usually a clear glass or acrylic sheet with one surface painted using a translucent pigment, the coated surface becoming the image forming surface for a picture projected on the screen. The techniques used for installing these rear projection screens were the same as the techniques used to install a display window in a store. With the advent of video projectors, an entirely new type of screen had to be developed because the old rear projection diffusion type screen would not function properly in this mode.
These new screens are called optical rear projection screens. They are generally two types of these screens : (a) a one piece screen consisting of two elements (fresnel lens and lenticular lens) or (b) a two piece screen consisting of three elements (fresnel lens, gathering lens and lenticular lens). In either case, the screen comprises more than one lens. These screens require new mounting techniques to satisfy the dimensional accuracy made necessary by the need for precise optical alignment of all the elements. Also, these new optical screens are made of a cast acrylic and are very lightweight compared to their glass predecessors. Therefore, since acrylic is sensitive to temperature and humidity changes, the manner of mounting the screen must accommodate the movement of the screen resulting from these properties.
Prior methods of mounting the screens did not satisfactorily allow for alignment adjustment of the screen to the projector. Additionally, these methods caused unnecessary strains and stresses on the screen which caused cracks in the screen. This has created a need for a new way to hang these optical screens which takes into account their special needs and characteristics.
Additionally, prior methods of holding two piece screens together have been unsuccessful since the screens were susceptible to misalignment and separation which caused the image to appear out of focus in the areas of the screen where the two pieces had separated from each other. This is particularly disadvantageous when computer text is displayed on the screen.
For example, the prior methods of holding the two pieces of the screen together required taping the pieces together along the four edges. However this method was unsuccessful since the screens separated. The front piece (the lenticular lens portion of the screen), which faces the audience, it is generally much thinner and more flexible than the back piece (the fresnel lens portion), which faces the projector. When the two pieces are cut to size, the front piece is generally made one or two millimeters shorter than the back piece. The front piece is also more flexible than the back piece, and therefore when the two pieces are taped together, it is assumed that the shorter front piece will be held against the face of the back piece. However, in the prior methods this does not happen, and the front slips down slightly to rest along its bottom edge and thereby buckles of its own weight, causing undesired separation between the two surfaces. The need exists for a way to mount the two pieces of the screen together which will prevent such slippage, buckling and subsequent separation.